Why is that women's leg hair grow faster than thigh hair?

Well not exactly, the impact of shaving is nowhere near this much, that's another myth. It's true that sometimes thighs are not shaved at first to not add too much at once to the routine of a teen which is new to shaving, but mainly because hair is typically softer there and despite the hair might get a bit worse after a while, though it's questionable how much is due to shaving, it usually stays softer than on lower legs anyway.